Individual flowering phenology shapes plant–pollinator interactions across ecological scales affecting plant reproduction

Abstract The balance of pollination competition and facilitation among co‐flowering plants and abiotic resource availability can modify plant species and individual reproduction. Floral resource succession and spatial heterogeneity modulate plant–pollinator interactions across ecological scales (individual plant, local assemblage, and interaction network of agroecological infrastructure across the farm). Intraspecific variation in flowering phenology can modulate the precise level of spatio‐temporal heterogeneity in floral resources, pollen donor density, and pollinator interactions that a plant individual is exposed to, thereby affecting reproduction. We tested how abiotic resources and multi‐scale plant–pollinator interactions affected individual plant seed set modulated by intraspecific variation in flowering phenology and spatio‐temporal floral heterogeneity arising from agroecological infrastructure. We transplanted two focal insect‐pollinated plant species (Cyanus segetum and Centaurea jacea, n = 288) into agroecological infrastructure (10 sown wildflower and six legume–grass strips) across a farm‐scale experiment (125 ha). We applied an individual‐based phenologically explicit approach to match precisely the flowering period of plant individuals to the concomitant level of spatio‐temporal heterogeneity in plant–pollinator interactions, potential pollen donors, floral resources, and abiotic conditions (temperature, water, and nitrogen). Individual plant attractiveness, assemblage floral density, and conspecific pollen donor density (C. jacea) improved seed set. Network linkage density increased focal species seed set and modified the effect of local assemblage richness and abundance on C. segetum. Mutual dependence on pollinators in networks increased C. segetum seed set, while C. jacea seed set was greatest where both specialization on pollinators and mutual dependence was high. Abiotic conditions were of little or no importance to seed set. Intra‐ and interspecific plant–pollinator interactions respond to spatio‐temporal heterogeneity arising from agroecological management affecting wild plant species reproduction. The interplay of pollinator interactions within and between ecological scales affecting seed set implies a co‐occurrence of pollinator‐mediated facilitative and competitive interactions among plant species and individuals.

ity in floral resources, pollen donor density, and pollinator interactions that a plant individual is exposed to, thereby affecting reproduction. We tested how abiotic resources and multi-scale plant-pollinator interactions affected individual plant seed set modulated by intraspecific variation in flowering phenology and spatio-temporal floral heterogeneity arising from agroecological infrastructure. We transplanted two focal insect-pollinated plant species (Cyanus segetum and Centaurea jacea, n = 288) into agroecological infrastructure (10 sown wildflower and six legume-grass strips) across a farm-scale experiment (125 ha). We applied an individual-based phenologically explicit approach to match precisely the flowering period of plant individuals to the concomitant level of spatio-temporal heterogeneity in plant-pollinator interactions, potential pollen donors, floral resources, and abiotic conditions (temperature, water, and nitrogen). Individual plant attractiveness, assemblage floral density, and conspecific pollen donor density (C. jacea) improved seed set. Network linkage density increased focal species seed set and modified the effect of local assemblage richness and abundance on C. segetum. Mutual dependence on pollinators in networks increased C. segetum seed set, while C. jacea seed set was greatest where both specialization on pollinators and mutual dependence was high. Abiotic conditions were of little or no importance to seed set. Intra-and interspecific plant-pollinator interactions respond to spatio-temporal heterogeneity arising from agroecological management affecting wild plant species reproduction. The interplay of pollinator interactions within and between ecological scales affecting seed set implies a co-occurrence of pollinator-mediated facilitative and competitive interactions among plant species and individuals.

K E Y W O R D S
agroecological infrastructure, Centaurea, network structure, plant-insect community, seed set, spatio-temporal heterogeneity

| INTRODUC TI ON
Sexual reproduction in outcrossing plant species contributes to genetically diverse populations and reduces inbreeding risks (Eckert et al., 2010). Seed set is one component of plant reproductive success that results from the plant's physiological state and efficient conspecific pollen (= genes) transfer, which is facilitated by foraging insects in many flowering plant species (Ollerton et al., 2011).
Physiological investment in reproduction by the plant individual (flowers and seeds) relies on the availability of water, energy, and nutrients, with any deficit likely to affect the physiological capacity for seed production (Akter & Klečka, 2022;Guilioni et al., 2003).
Furthermore, intra-and interspecific plant competition for these abiotic resources can divert investment from sexual reproduction and modify the size of floral displays used to attract the pollinators needed for cross-pollination (Akter & Klečka, 2022).
Plants may also compete for insect pollination because plantpollinator interactions typically occur in multi-species assemblages (Bascompte & Jordano, 2007). Indirect plant-plant interactions through pollinator sharing can reduce seed set due to interspecific pollen transfer that disrupts conspecific pollination (Arceo-Gómez & Ashman, 2014; Morales & Traveset, 2008). Conversely, facilitation of plant reproductive success can also occur in diverse co-flowering assemblages when greater pollinator densities or diversity increase flower visitation rates or provide complementarity and redundancy in pollination services (Blüthgen & Klein, 2011;Ghazoul, 2006;Hegland, 2014). Moreover, pollinator foraging behaviors vary with the relative quality, abundance, and accessibility of pollen or nectar due to the spatio-temporal turnover in flowering plant assemblages (Gallagher & Campbell, 2020;Jha & Kremen, 2013;Lázaro et al., 2009). This combination of foraging plasticity and spatio-temporal floral heterogeneity can determine the level of pollination competition or facilitation among plant individuals and species to alter plant reproductive outcomes.
This complexity of multispecies interactions and ecological processes influencing plant reproduction can be understood as a plant-pollinator network (Bascompte & Jordano, 2007). Variation in the organization and strength of species interactions in a network reflects community composition, species coevolution, and ecological processes like competition and resource partitioning (Junker et al., 2013;Magrach et al., 2021;Vázquez et al., 2009).
Greater network linkage density (encompassing the overall species richness and frequency of interactions) might affect pollination processes by increasing overall flower visitation rates (Akter et al., 2017) or by offering potential complementarity or redundancy in pollinators (Blüthgen & Klein, 2011;Ghazoul, 2006;Magrach et al., 2021;Venjakob et al., 2016). A high level of plant species specialization in pollinator interactions (d′-Blüthgen et al., 2006(d′-Blüthgen et al., , 2008Dormann et al., 2009) implies that pollen transfer relies on relatively few pollinator species in the multispecies network, which, under adaptive foraging, may dictate the potential for floral constancy, conspecific pollen transfer, and subsequent seed set (Valdovinos et al., 2013). This potential would be greatest where there is a strong mutual dependence between pollinator and plant species in the network re-enforcing conspecific pollen transfer (Bascompte et al., 2006;Vázquez et al., 2007). However, an insect species dominating visitation to a particular plant species but with lower depen- Consequently, variation in plant-pollinator network structure has the potential to indicate and influence pollination efficiency and plant reproduction, but relatively few studies have examined this relationship (Arceo-Gómez et al., 2020;Arroyo-Correa et al., 2021;Lázaro et al., 2020;Magrach et al., 2021;Theodorou et al., 2017;Vanbergen et al., 2014). Moreover, reports are idiosyncratic with neutral (Theodorou et al., 2017) or positive (Arroyo-Correa et al., 2021;Lázaro et al., 2020) effects of network structure on seed production.
Plant-pollinator interactions are filtered by the combination of organism traits and environmental conditions at different levels of ecological organization (Arroyo-Correa et al., 2021;Lázaro et al., 2009).
Cost-benefit dynamics governing mutualistic plant-insect relationships (Bronstein, 1994) mean that reproductive outcomes are often unbalanced among plant species and individuals (Mesgaran et al., 2017). Furthermore, the occurrence of distinct assemblages of species interactions at different ecological scales (organism to community) may have complementary or opposing effects on plant pollination and reproduction (Hegland, 2014;cf. specialization-Brosi, 2016). For instance, plant species that are scarce within a species-rich assemblage may experience dilution of pollinator visits, reduced pollen transfer, and seed production (Evans et al., 2017).
Conversely, large intraspecific or interspecific floral displays can increase plant-mating opportunities through the overall attraction of floral visitors (Akter et al., 2017;Hegland, 2014). However, a large floral display in an individual plant may increase the risk of geitonogamous pollen transfer (self-pollination), which can increase risks of inbreeding and in self-incompatible species reduce seed production (Akter et al., 2017;Eckert et al., 2010;Karron & Mitchell, 2012).
Apart from spatial effects, flowering phenology (i.e., the timing and duration of flowering period) is a species trait that influences plant reproduction. Inter-and intraspecific phenological variation filters the precise assemblage of interacting species that a plant individual is exposed to during the temporal succession of plants and pollinators, both in the immediate local assemblage and the wider community across the landscape (Arroyo-Correa et al., 2021;CaraDonna & Waser, 2020;Gallagher & Campbell, 2020;Rafferty & Ives, 2012). The degree of overlap in the timing and duration of flowering periods, within and between species, therefore modulates the level of insect-mediated conspecific pollen transfer and the balance of interspecific interactions (competition, interference, or facilitation) at different ecological scales affecting pollination services and plant seed set (Kovács-Hostyánszki et al., 2013).
Pollinators and pollination services face anthropogenic threats with conventional intensive agricultural management the foremost worldwide (Dicks et al., 2021;Potts et al., 2016). Ecological intensification of agriculture is one alternative management model to reduce the negative impacts of agriculture and respond to global change while maintaining food production (Vanbergen et al., 2020).
Ecologically intensive practices include the use and rotation of diverse crops and existing or restored agroecological infrastructures (e.g., semi-natural habitats, sown wildflower, or grass strips) to harness ecosystem services, like pollination, in support of agriculture 5. Seed set will be modulated by the interplay of plant-pollinator relationships occurring within and between ecological scales due to the mobility of insects transferring pollen.

| Focal plant species
Cyanus segetum Hill, 1762, and Centaurea jacea L., 1753 [Asteraceae], were chosen as phylogenetically related herbaceous species with contrasting flowering phenology and with populations on the study site. C. segetum is an annual segetal species with individuals flowering from May to July. C. jacea is a common perennial of grassy environments flowering in late summer (July-October in Burgundy, France; Tison & de Foucault, 2014). Both species require insect pollinators (C. jacea-self-incompatible; C. segetum-pseudo-selfcompatible) and provide high-quality pollen and nectar resources for flower-visiting insects (Bellanger et al., 2014;Hicks et al., 2016;Ouvrard et al., 2018;Steffan-Dewenter et al., 2001). Their prolonged flowering periods (Monticelli et al., 2022) present intraspecific phenological variations that dictate individual plant exposure to plant-pollinator interactions.

| Abiotic environment-influencing plant physiological capacity to invest in seed production
At the farm scale, we calculated the mean temperature (°C) and the mean precipitation (mm) for each individual's flowering period until harvested using daily records from an automated meteorological station at the experimental farm (Equations S1). At the individual plant scale, we used the foliar N content (%) of each plant individual as an indicator of its physiological state and a proxy for the biochemical resources available for investment in reproduction (Wang et al., 2018).
One leaf sample per individual (~5 g) was collected before the flowering period of each species (C. segetum-mid-May; C. jacea-early July). After oven drying (40°C) and milling (diameter ≤80 μm), the N content in 4-6 mg of foliar tissues (%) was quantified using a Thermo Scientific FLASH 2000 Organic Elemental Analyzer™.

| Reproductive development and seed set of focal plant individuals
We counted the open and wilted composite flowers, floral buds, and fruits produced by each focal plant individual at monthly intervals. Combined with insect visitation data, these measurements allowed us to estimate the flowering period of each individual plant (precision to the week). At the end of the flowering period (i.e., C. segetum-mid-July; C. jacea-early-September), all surviving focal plants were harvested. We counted the total number of seeds produced per individual (hereafter "seed set") and the total number of floral heads (fruits with or without seeds) as a measure of the size of the total individual floral display.

| Plant-pollinator interactions from individual to local assemblage scales
We quantified plant-pollinator interactions to each focal plant species (plot -Table S2) and in the local floral assemblage (transect -Tables S2 and S3)  We also calculated the local species richness of potential pollinators of C. segetum or C. jacea during each individual's flowering period.
We defined potential pollinators as insects observed visiting focal or non-focal C. segetum or C. jacea, respectively, in the surveyed agroecological infrastructure (Table S2) The dependence of the plant species (c) on pollinator p and reciprocally (p on c) were multiplied and the products were summed across all pollinator species to give the total mutual dependence of each focal plant species. The interactions were weighted according to the total observation frequency of pollinator species to exclude pollinator species only observed once (Blüthgen et al., 2008). This index varies between 0 (weak mutual dependence) and 1 (strong mutual dependence = pairwise mutualism or perfect nestedness), with higher values expected to reflect increased efficiency of focal plant pollen transfer (Vázquez et al., 2007).

| Statistical analyses
We used one generalized linear mixed model per species (GLM, "lme4") to explain the intraspecific variations in seed set between surviving focal individuals of C. segetum (n = 144) and C. jacea (n = 105) fitting a negative binomial distribution to control for overdispersion. We fitted "plot" as a random effect to account for the spatial dispersion of the replicates and unmeasured microsite conditions (singularity meant we dropped this random effect in the C. jacea model to avoid overfitting). This ensured high precision in the estimation of the abiotic context and assemblages of plant-pollinator interactions the individual was exposed to at the level of the individual plant, local assemblage, and the interaction network of agroecological infrastructure across the farm (Table S4). insects from 26 species and 88 insects from 24 species (Table S2).

| Abiotic variables influencing focal plant seed set
Only mean temperature over the flowering period related negatively and positively to seed set in C. segetum and C. jacea, respectively (Tables 1 and 2). Neither precipitation nor foliar N content were selected in the best models.

| Plant-pollinator interactions influencing seed set at focal plant and local assemblage scales
The   Figure 1e), but C. segetum was unaffected.
Although for both focal plant species, the species richness of potential pollinators active in the local floral assemblage was among the main fixed effects predicting seed set, its effect was relatively weak compared to other parameters (Tables 1 and 2; Figure 1c,f).
An albeit weak statistical interaction (floral richness × potential pollinator richness) indicated that the response of C. segetum seed set to the species richness of potential pollinators foraging in the local assemblage was negative when situated in florally poor local assemblages, but positive in most species-rich floral assemblages (Table 1; Figure 1c).

| Properties of plant-pollinator network structure in agroecological infrastructure affecting seed set
As a main effect, network linkage density positively affected seed set in both focal species (Tables 1 and 2 (Table 1; Figure 2b). In the more densely linked networks, the local species richness of potential pollinators had a positive relation to C. segetum seed set, with only a negative relationship at the lowest level of linkage density (Table 1; Figure 2c).
C. jacea specialization in pollinator interactions (d′) had a negative impact on its individual seed set as a main fixed effect (Table 2), but C. segetum was unaffected. As a main fixed effect, the effect of total weighted MD between C. segetum and its pollinators had a positive influence on seed set (Table 1; Figure 2d).  Floral density (log) × linkage density (log) −0.14 ± 0.05 −2.93 <.01 Note: The GLMM (negative binomial) was derived from AIC-based multi-model selection, and predictors were present in >50% of the 22 best models (ΔAIC <2). Total variance explained (R 2 ) by marginal (fixed effects) and conditional (fixed + random) predictors, and the plot random effect (σ 2 ) are cited. Predictors were log-transformed where required to account for non-linear relationships with the log of seed number.  Note: The GLM (negative binomial) was derived from AIC-based multi-model selection and predictors were present in >60% of the 15 best models (ΔAIC <2). The random effect was dropped due to model singularity and to avoid overfitting; therefore, the total variance explained (R 2 ) is due solely to the fixed effects. Predictors were log-transformed (n + 0.0001) where required to account for non-linear relationships with the log of seed number. Nitrogen and water availability were predicted to affect the plant's capacity to invest in reproduction (Akter & Klečka, 2022;Guilioni et al., 2003), but they were unimportant in this case, perhaps because the farm-scale (125 ha) environmental gradients in abiotic resources were insufficiently strong to affect seed production. Although the environmental temperature at the farm scale was selected in the final model for both species, it was generally a less important determinant of seed production than floral assemblages or plant-pollinator interactions.
The size of the individual floral display is an important determinant of insect visitation rate, pollination, and seed set (Akter et al., 2017;Karron & Mitchell, 2012). As predicted, seed set for both focal species was related positively to the individual plant's relative attractiveness, an index integrating the plant's capacity to invest in a large floral display, the attraction via the floral display of conspecific neighbors, and the corresponding pollinator visitation rate (Akter et al., 2017). Individual C. jacea with larger floral displays also had greater success at attracting and concentrating visitation with subsequent benefits for seed set when situated within assemblages with greater interspecific floral densities. Along with the direct relationship between focal plant seed set and floral density in the local assemblage, this suggests the overall attraction of pollinators to a dense floral community (Hegland, 2014) whose plastic foraging behaviors then facilitated the transfer of pollen among focal plants (Jha & Kremen, 2013;Petanidou et al., 2008).
A greater density of potential conspecific pollen donors in the local assemblage enhanced individual seed set of C. jacea but not C.
segetum. This difference is possibly due to the relative population size of the two focal species (Table S4). Although both focal species were part of the sown wildflower seed mix, C. jacea only flowers after a year of vegetative growth, whereas C. segetum is annual (Nitschke et al., 2010;Tison & de Foucault, 2014). Consequently, the availability of potential pollen donors was more limiting for C. jacea with only few naturally occurring individuals on the farm providing a source of outcross pollen in addition to the transplanted focal individuals, whereas in the sown mixtures there were readily available pollen donors for C. segetum (Eckert et al., 2010).
The interplay between pollinator and floral species richness in the local assemblage further affected C. segetum reproduction, although relatively weakly, compared to other predictors. C. segetum seed set tended to decrease or increase with increasing local species richness of potential pollinators when assemblage floral richness was low or high, respectively. A potential explanation is that in species-poor floral assemblages, although plant competition for F I G U R E 3 Seed yield of focal C. jacea individuals in relation to (a) network linkage density and (b) d′ specialization × mutual dependence between C. jacea and its pollinators. Fitted lines (± CI) are partial residuals from GLMs accounting for other fixed effects. The scatter plots show the distribution of the raw data. One point was deleted from graph A (seed set = 15,769) for better visualization (but not from the calculation of marginal effects).
The structure of plant-pollinator networks in agroecological infrastructure across the farm also influenced the reproduction of the individual focal plants during their respective flowering periods.
Linkage density provides a metric of the overall species richness and frequency of interactions in the network (Dormann et al., 2009).
Greater network linkage density had a strong positive influence on individual C. segetum seed set (but less strongly for C. jacea).
Furthermore, the species richness of potential pollinators and floral density in the local assemblage most benefited C. segetum seed set when linkage density was higher, showing how spatio-temporal heterogeneity across ecological scales influenced plant reproduction (Hegland, 2014;Kovács-Hostyánszki et al., 2013). Together, these results suggest that the reproduction of these focal species (particularly C. segetum) visited mostly by generalist pollinators (Table S2) benefited from being embedded within a wider species-rich network with high flower visitation rates. This might be due to adaptive foraging (Valdovinos et al., 2013), trait matching (Garibaldi et al., 2015), and/or species complementarity or redundancy (Blüthgen & Klein, 2011;Venjakob et al., 2016;Woodcock et al., 2019)  We also predicted that seed set would be modulated by the level of focal plant specialization on pollinators (d′) or MD in the farm-scale network of agroecological infrastructure through gains in conspecific pollen transfer between spatially separated plants (Bascompte et al., 2006;Valdovinos et al., 2013;Vázquez et al., 2007). A greater level of MD between the focal species and their pollinators, indicating higher constancy of pollinator interactions on the focal plants compared to other plant species, contributed to increase seed set (Morales & Traveset, 2008;Vázquez et al., 2007). For C. jacea, the positive effect of MD was augmented when specialization on pollinators (d′) was high. Pollinator networks are typically nested, meaning there is a high reliance of specialist plants on generalist pollinators foraging on many floral species (Bascompte & Jordano, 2007), potentially diluting conspecific pollen transfer (Arceo-Gómez et al., 2020;Lázaro et al., 2020).
Under adaptive foraging, those generalist pollinators may minimize competition by concentrating on their specialist plant partners (Valdovinos et al., 2013). Concomitantly high levels of MD and d′ may reflect a level of adaptive foraging that overcame the negative influence of nestedness on conspecific pollen transfer. Whereas when C. jacea specialization was low, there was an inverse relationship between MD and seed set which may be consistent with interspecific pollinator interferences resulting from high pollinator activity on C. jacea (Greenleaf & Kremen, 2006).  (Brosi, 2016;Hegland, 2014;Kovács-Hostyánszki et al., 2013;Mesgaran et al., 2017).
A caveat to our study is that we only detect correlative patterns in pollinator and plant biodiversity and interactions, which we interpret according to known ecological processes (e.g., competition vs facilitation). Although we predicted and controlled for the effect of intra-and interspecific phenology on plant-pollinator interactions, the two plant species differ in other traits (e.g., life cycle; Tison & de Foucault, 2014) that may also affect their interaction with pollinators. Additional field experiments that manipulate these processes (e.g., competition and temporal turnover) or assemblage structure (e.g., diversity, trait, or functional group structure) are needed to verify the precise mechanisms that produced the observed patterns at different ecological scales (Magrach et al., 2021).
Our results highlight how the balance of intra-and interspe-

ACK N OWLED G M ENTS
We acknowledge the financial support of Ecole Doctorale-

CO N FLI C T O F I NTE R E S T
The authors declare no conflict of interest.